A sensor for detecting and signalizing a displacement of a displacement measurement object is referred to as a displacement sensor, and especially, a displacement sensor which uses a differential signal due to a change in a mutual inductance according to a change in a position of an object is referred to as an inductive displacement sensor. In general, an inductive displacement sensor includes coils causing a mutual inductance change when a position of a displacement measurement object is changed, a sensor driving means for supplying a drive voltage, a differential circuit for outputting a differential signal according to a change in the mutual inductance, and a detection circuit for detecting amplitude information from a differential signal obtained from the differential circuit.
Magnetic bearings for supporting a rotary body by using a magnetic force without directly contacting the rotary body are widely employed in general industrial products. A magnetic bearing has an unstable property in a suction direction of pulling a rotary body or in a vertical direction according to its technology. In order to solve the unstable element and successfully realize the magnetic bearing, a control technology for detecting a displacement of a rotary body and regulating a current accordingly to support the rotary body at a predetermined position is important. That is, since a precision and an accuracy of a magnetic bearing vary according to a performance of a displacement sensor for detecting a change in position of a rotary body, a technology of improving a bandwidth of the displacement sensor to improve the precision and accuracy of the magnetic bearing is necessary.
As displacement sensor for detecting a change in position of a rotary body of the magnetic bearing, inductive displacement sensors for measuring a displacement of the rotary body through detection of a change in a mutual inductance generated according to a change of position of the rotary body are widely utilized to control a rotary body of a magnetic bearing. An inductive displacement sensor according to the related art obtains a difference between an electrical signal produced in one coil through a separate differential circuit and a signal produced in a coil compared with the coil after the same power source is supplied to the coils, and thus a minimum of one differential circuit needs to be realized for one displacement sensor.
An inductive displacement sensor according to the related art using a displacement detection circuit through half-wave rectification or full-wave rectification inevitably generates ripples having a harmonic component of a drive frequency in a displacement signal detecting process because of a property of the detection circuit. As a low pass filter having a low interruption frequency is used to remove the ripples, an output displacement signal has a frequency lower than a drive frequency. The low frequency of the output displacement signal lowers an accuracy and a precision of the displacement sensor.
In addition, the problem of the inductive displacement sensor according the related art lowers an accuracy and a precision of control of a magnetic bearing.